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Rien

Cp to CR Throttle body modification

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Hello,

 

I wonder, if there is someone who really knows why Triumph the CP manifolds has changed to CR.

 

What is function of the bulge, in the channel, and the front balance pipe.

 

For the balance pipe, I think it is to flatten the depression peaks caused by the 311399 camshaft.

The bulge in the channel, I think is to make sure that the injected fuel, are better mixed.

 

What are you think?

 

Thanks,

Rien

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Good question to open a can of worms!

 

The second air link between the 6 inlet runners is for not

needing that perfect balancing as that was often a problem of the CP-PIs.

 

Any link between the runners creates bad pulsations that change the amount of air

sucked into that cylinder at that cycle.

The hotter the cam the more these pulsations will occure and they can be found most at

idle and become less at higher revs.

 

The worst one can do is to connect the runners fully and use a hot cam.

Lumpy idle and bad torque at low revs will be the result and should be avoided.

Could earlier be found on race cars group 1B where modifications are allowed but

the stock manifold must be kept. As manifold was designed with single throttle for

street use the car idled badly with the hot cams.

 

By the way this separation is one of the reasons why WEBER DCOE are so famous and even

race engines idle acceptable and pull from idle nicely.

 

It is not possible to smooth the pressure peaks in the manifold so still the other connection

of the CR is responsible for creating the pressure signal to switch the MU. In that line the peaks are

smoothed out by connecting relatively small lines into the manifold and have a small volume in the

connecting pipe. So for one manifold that peaks there are five happy to give air into that line

to smooth. Easy trick we used former for MegaSquirt1 that did not have those facilities to smooth

the signal electronically.

 

As CR got a milder cam Triumph could go that way easily without sacrifice idle quality

and get better behaviour with slightly misbalanced throttles.

 

The other thing they established at CR was the progressive linkage that opens

throttles less from idle and more towards full throttle. Also amount of parts to wear

are dramatically reduced to keep the setup for longer period.

 

The D-shape should increase low end torque and make emissions better

but I think thats bullshit. I cut that off and use the hotter cams anyway for

good top end. That "D" that can be seen in the inlet viewed from the valve

should give a sharp edge to cut the pulsation going back from valve pressing

the air backwards. Never found this on a sporty engine so I do not rely on it.

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Good question to open a can of worms!

 

The second air link between the 6 inlet runners is for not

needing that perfect balancing as that was often a problem of the CP-PIs.

 

Any link between the runners creates bad pulsations that change the amount of air

sucked into that cylinder at that cycle.

The hotter the cam the more these pulsations will occure and they can be found most at

idle and become less at higher revs.

 

The worst one can do is to connect the runners fully and use a hot cam.

Lumpy idle and bad torque at low revs will be the result and should be avoided.

Could earlier be found on race cars group 1B where modifications are allowed but

the stock manifold must be kept. As manifold was designed with single throttle for

street use the car idled badly with the hot cams.

 

By the way this separation is one of the reasons why WEBER DCOE are so famous and even

race engines idle acceptable and pull from idle nicely.

 

It is not possible to smooth the pressure peaks in the manifold so still the other connection

of the CR is responsible for creating the pressure signal to switch the MU. In that line the peaks are

smoothed out by connecting relatively small lines into the manifold and have a small volume in the

connecting pipe. So for one manifold that peaks there are five happy to give air into that line

to smooth. Easy trick we used former for MegaSquirt1 that did not have those facilities to smooth

the signal electronically.

 

As CR got a milder cam Triumph could go that way easily without sacrifice idle quality

and get better behaviour with slightly misbalanced throttles.

 

The other thing they established at CR was the progressive linkage that opens

throttles less from idle and more towards full throttle. Also amount of parts to wear

are dramatically reduced to keep the setup for longer period.

 

The D-shape should increase low end torque and make emissions better

but I think thats bullshit. I cut that off and use the hotter cams anyway for

good top end. That "D" that can be seen in the inlet viewed from the valve

should give a sharp edge to cut the pulsation going back from valve pressing

the air backwards. Never found this on a sporty engine so I do not rely on it.

Hi Andreas,

Thats a lot to think about, I am just finisched rebuilding a 2.5 PI engine for a friend, al CR parts and spec's changed to CP ecxept the throttle bodies, do I understand that you recomment that I blind off the inner balance tube, turn out the bulges and place the bleed valve to the outer tube?

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Hi Rien, I have a CR with 290 degree Bastuck prepared

and it idles well and pulls from 1300rpm.

 

So it seems that it works up to that properly and you could

check that first. If there is bad idle you know what might help

and blind it later.

 

I do not know why the air bleed valve walked to the inner tube.

All I can say is that I am not a friend of the double valves on left

and right side of the outer tube. Mine worked perfect without that.

 

If you want to do something real advantageous you could swap to

ball bearings in the manifold. Its more precise and turns easier

especially when manifold pressure sucks the throttle plates.

 

 

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